The invention relates to a method of making a tube of thermoplastic material comprising an inner wall and an outer wall being interconnected by partitions, thereby forming longitudinal channels which are interspaced along the circumference, said channels having a rounded cross-section.
By a rounded cross-section is meant either a circular or an ellipsoidal cross-section, as well as other types of cross-sections with four bent walls and rounded corner transitions.
A plastic tube of this type, the wall of which comprises longitudinal channels presenting a circular cross-section, is known per se. Said tubes are manufactured by extruding a plasticized thermoplastic material, such as polyvinylchloride, through an extrusion nozzle. These longitudinal channels are regularly subdivided along the circumference and the distance between two adjacent channels is preferably equal to the distance of the outer surface of the tube to the nearest wall of a channel.
Said known tubes have the drawback that, when a sharp tool hits the front side of the plastic tube, a tear will be formed and this tear will then increase and extend along the whole length of the tube, so that the tube is useless.
Said tears are due to the fact that, the outer side of an extruded plastic tube is cooled down, whereafter, in a later stage, the inner wall, being interconnected with the outer wall, shrinks owing to cooling. Owing thereto, pressure tensions will occur in the outer wall, thereby causing tensile stresses in the inner wall of the tube.
Though attempts have been made to obviate this difficulty by using very specific types of plastics, said difficulties cannot be eliminated in this way.